This invention relates to a semiconductor device, in which the breakdown voltage is increased, and a method of manufacturing the same.
FIGS. 1 and 2 show examples of high breakdown voltage semiconductor devices, i.e. transistors having a breakdown voltage of 500 V or higher, which are well known in the art.
FIG. 1 shows a vertical npn bipolar transistor. In this Figure, reference numeral 41 designates an n.sup.+ type collector region, 42 an n-type high resistivity collector region, 43 a p-type base region, 44 an n.sup.+ -type emitter region, 45 a base electrode, and 46 an emitter electrode. Such a transistor has a vertical structure, and it permits high breakdown characteristics to be obtained owing to p-type guard ring region 47 surrounding p-type base region 43.
However, the base-emitter breakdown voltage (BV.sub.CEO) depends on the current amplification factor of the transistor and is unstable. Further, rupture of the element is liable to result from heat-based secondary yield phenomenon. Therefore, problems arise with reliability. Further, guard ring area 47 increases the element area.
FIG. 2 shows a double diffusion type MOS transistor (D-MOS). In this Figure, reference numeral 51 designates an n.sup.+ -type drain region, 52 an n-type drain high resistivity region, 53 a p-type back gate region, 54 an n.sup.+ -type source region, 55 a gate electrode, 56 a source electrode, and 57 a p-type guard ring region.
A MOS transistor as shown in FIG. 2 has a double diffusion structure, wherein it is possible to obtain high breakdown characteristics. However, because the MOS transistor as shown in FIG. 2 is a vertical element structure, the width l1 of n-type high resistivity drain region 52 is not made accurately. Therefore, if fluctuations are produced in the "on" resistance of MOS transistor, the "on" resistance is increased due to the influence of a parasitic junction field-effect transistor (i.e., junction FET) produced in n.sup.- -type high resistivity drain region 52 between opposed p-type back gate regions 53. Therefore, there are problems in characteristics other than the breakdown characteristics. Even with such a MOS transistor, guard ring region 57 is necessary, so that the area of the element is increased.
FIG. 3 shows a vertical MOS transistor having open-drain structure. In this Figure, reference numeral 61 designates a p-type substrate, 62 an n-type high resistivity drain region, 63 a p-type back gate region, 64 an n-type source region, 65 an n.sup.+ -type drain region, 66 a gate electrode, 67 a source electrode, and 68 a drain electrode.
MOS transistor as shown in FIG. 3 also has high breakdown characteristics because of the structure noted above. However, because a depletion layer occurs from p-type substrate 61 into n.sup.- -type high resistivity drain region 62, it is necessary to form the width l2 of high resistivity drain region 62 sufficiently wide. High resistivity drain region 62 is formed by means of the epitaxial growth process. Therefore, in order to sufficiently increase the width of region 62, a long time is required for the epitaxial growth. Further, in this transistor source electrode 67 is connected to p-type substrate 61 via p-type back gate region 63, and source electrode 67 is held at the substrate potential. Therefore, with this transistor the output signal can be taken out only from drain electrode 68, and the use is limited.